I need to replace the patio doors on the south side of my house. What type of glass would you recommend?
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I have an existing solar house 30 miles south of Boston. I need to replace the sliders (patio doors) on the south side of the house for passive gain. What type of glass would you recommend: plain glass or low-E argon-gas type panes? Will the low-E prevent too much sun and thereby diminish the solar gain?
What you are looking for is the lowest U-value while keeping as high a SHGC (Solar Heat Gain Coefficient) as possible. This information can be found on the NFRC label on nearly every window available for purchase.
In order to get an acceptable U-value, you will need an argon-filled, double-pane glass. What you want to be avoiding, though, are most manufacturer-specific proprietary coatings that reduce the SHGC further without improving the U-value. Also be aware that, unfortunately, the SHGC must be 0.3 or less if you hope to get the federal tax credit. However, some of the better dual-pane glass available has a U-value of about 0.25 with an SHGC of 0.42, which would be excellent for your application.
Another important area to consider is heat loss through infiltration, that is, a bad seal. Sliders, at least most American-made products, slide in their seal, and therefore the seal cannot be very tight. I therefore advise anyone wanting to build a more sustainable home with domestic window products to consider french doors and casement windows, as you can get much better sealing units.
To circumvent this problem for my clients we import European windows on all our projects. That is not worth the effort if you just want to replace a few units, but there are also some European manufacturers who distribute on the U.S. market directly if you insist on sliders. These sliders lift out of their seal or retract the seal before you slide the door, lowering air infiltration significantly.
Finally, remember to protect your home from overheating in summer too, especially if you increase the size of southern or western windows. Depending on your specific needs, overhangs, deciduous trees or shades may be a good solution to prevent solar gain.
For more information:
Read David Edwards's Q&A "Are there any nontoxic, non-offgassing replacement windows available? Also, how does one identify nontoxic sheetrock in stores?"